In a 3rd Generation Partnership Project Long Term Evolution (3GPP LTE) system typifying a 4th generation mobile communication system, orthogonal frequency division multiple access (OFDMA) serving as an orthogonal multiple access (OMA) scheme is used in a downlink. In the OFDMA, in the case of allocating a common time resource to users (for example, terminals) serving as scheduling processing targets, sub-bands that do not interfere with one another (in other words, sub-bands orthogonal to one another) are allocated to respective users, as illustrated in FIG. 1. FIG. 1 is a diagram made available for explaining orthogonal multiple access. In particular, in FIG. 1, resources are allocated to two users, namely, a user #1 (UE#1) and a user #2 (UE#2). If signal-to-noise ratios (SNRs) of the users #1 and #2 are SNR1 and SNR2, respectively, and resource allocation rates (in other words, allocated bandwidths) of the users #1 and #2 are ρ1 and ρ2, respectively, a “capacity (hereinafter, called a “predicted throughput” or an “expected throughput” in some cases)” of each of the users may be expressed by the following Expression (1).Rk(OMA)=ρk log(1+SNRk), k=1,2  (1)
In contrast, in a 5th generation mobile communication system, a non-orthogonal multiple access scheme has been studied. In non-orthogonal multiple access, in the case of allocating a common time resource to users serving as “scheduling processing targets, sub-bands that interfere with one another (in other words, non-orthogonal sub-bands) are allocated to respective users. Namely, in non-orthogonal multiple access, a same radio resource having a same time and a same frequency is allocable to two or more terminals. In other words, as illustrated in, for example, FIG. 2, in a common sub-band, given amounts of power are distributed (allocated) to the user #1 and the user #2. FIG. 2 is a diagram made available for explaining the non-orthogonal multiple access. In a non-orthogonal multiple access system, for example, a communication device on a receiving side has a function (in other words, a successive interference canceller (SIC) function) of cancelling a signal, addressed to another communication device and assigned to the same resource as that of the device of own, from a reception signal and performing demodulation processing and decoding processing on the reception signal after the cancellation processing.
It is assumed that, as two users serving as targets of non-orthogonal multiplexing, for example, the user #1, which is located near a base station and whose SNR is high, and the user #2, which is located away from the base station and whose SNR is low, are selected. Since the SNR of the user #2 is low, the number of modulation levels and a coding rate (in other words, a modulation and coding scheme (MCS)), applied to a signal addressed to the user #2, are lower than those applied to a signal addressed to the user #1. Therefore, it is possible for the user #1 to demodulate and decode the signal addressed to the user #2 with a high success probability. Accordingly, by cancelling the signal addressed to the user #2 from a reception signal, it is possible for the user #1 to easily remove interference from the signal addressed to the user 2#. In other words, if it is assumed that the SNR and allocated power of the user #1 are SNR1 and p1, respectively, the capacity of the user #1 may be expressed by the following Expression (2).R1(NOMA)=log(1+p1SNR1)  (2)
On the other hand, the signal addressed to the user #1 becomes interference to the signal addressed to the user #2. Accordingly, if it is assumed that the SNR and allocated power of the user #2 are SNR2 and p2, respectively, the capacity of the user #2 may be expressed by the following Expression (3).
                              R          2                      (            NOMA            )                          =                  log          ⁡                      (                          1              +                                                                    p                    2                                    ⁢                                      SNR                    2                                                                    1                  +                                                            p                      1                                        ⁢                                          SNR                      2                                                                                            )                                              (        3        )            
In other words, the signal addressed to the user #1 is a factor in reducing the capacity of the user #2. However, since the SNR of the user #2 is originally low, an influence on the throughput of the user #2 is high due to an interference noise of an another signal other than the signal addressed to the user #1. Therefore, the influence thereon due to interference of the signal addressed to the user #1 is low.
In this way, according to the non-orthogonal multiple access, it may be expected that the sum of the capacities of all the users serving as multiplexing targets (in other words, a “system capacity”) is improved compared with the orthogonal multiple access.
A technology of the related art is disclosed in Benjebbour A., Saito Y., Kishiyama Y., Li A., Harada A., and Nakamura T., “Concept and practical considerations of non-orthogonal multiple access (NOMA) for future radio access”, ISPACS 2013, November 2013.